1. Field of the Invention
The invention generally relates to handling Mobility Management (MM) back-offs, and more particularly, to apparatuses and methods for handling MM back-off timers in a per Public Land Mobile Network (PLMN) basis.
2. Description of the Related Art
In a typical mobile communications environment, a user equipment (UE), including a mobile telephone (also known as cellular or cell phone), a laptop computer with wireless communications capability, or a personal digital assistant (PDA) etc., may communicate voice and/or data signals with one or more service network. The wireless communications between the UE and the service networks may be in compliance with various wireless technologies, such as the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, Code Division Multiple Access 2000 (CDMA 2000) technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology, Worldwide Interoperability for Microwave Access (WiMAX) technology, Long Term Evolution (LTE) technology, LTE-Advanced technology, and others.
Take the Evolved Packet System (EPS) in compliance with the 3GPP TS 23.401 specification, v.10.2.0 (referred to herein as the TS 23.401 specification), the 3GPP TS 24.301 specification, v.10.1.0 (referred to herein as the TS 24.301 specification) for example. During an Attach procedure, as shown in FIG. 1, or a Tracking Area Update (TAU) procedure, as shown in FIG. 2, the attachment request or the TAU request may be rejected due to the Non-Access Stratum (NAS) level congestion control of a first Public Land Mobile Network (PLMN). In this case, the ATTACH REJECT message or the TRACKING AREA UPDATE REJECT message comprises an MM back-off timer to limit the attachment retry or TAU retry of the UE towards the first PLMN. Specifically, the UE can not initiate any MM procedure, including the Attach procedure, the TAU procedure, and the Service Request procedure, with the first PLMN until the MM back-off timer expires. After being rejected by the first PLMN and the MM back-off timer is started for the first PLMN, the UE may reselect to a second PLMN to initiate the Attach procedure or the TAU procedure. If, unfortunately, the second PLMN is also congested and rejects the attachment request or the TAU request from the UE, the UE may receive another MM back-off timer in the rejection message from the second PLMN. Correspondingly, the UE stops the MM back-off timer associated with the first PLMN and starts the MM back-off timer associated with the second PLMN, since only one MM back-off timer (i.e., the MM back-off timer associated with the currently selected PLMN) should be maintained by the UE. This may result in a situation where the UE reselects back to the first PLMN which may be very likely still congested since the time period for the initial MM back-off timer associated with the first PLMN has not passed yet. Thus, the NAS level congestion control of the first PLMN would be impaired, and the obtainment of wireless services by the UE would be prolonged.